1. Field of the Invention
The present invention generally relates to camera devices and a method of manufacturing the same, and more particularly, to a camera device having a photoelectric converting portion and a method of manufacturing the same.
2. Description of the Background Art
With a camera device made high in quality and in density, high sensitivity in a photoelectric converting portion of the camera device has been conventionally required. As for the camera device, a camera tube, a solid state image sensing device and the like are known.
FIG. 6 is a schematic cross sectional view showing a structure of a conventional camera tube. This is disclosed in, for example, Japanese Journal of Applied Physics, Vol. 28, No. 2, February 1989, pp. 178-186, or the like. FIG. 7 is an enlarged cross sectional view showing a structure of a target portion of the camera tube shown in FIG. 6. The target portion shown in FIG. 7 is disclosed in, for example, IEEE Electron Device Letters, Vol. EDL-8, No. 9, September 1987, p. 392 or the like.
Referring to FIGS. 6 and 7, the conventional camera tube includes a glass tube 101, a target portion 106 attached so as to hermetically seal an opening end of glass tube 101, a cathode 102 attached in glass tube 101 so as to oppose target portion 106 for generating an electron beam, an accelerating electrode 103 attached in glass tube 101 for accelerating the electron beam, a polarization coil 104 disposed along a circumference of glass tube 101 for polarizing the electron beam, and a focus coil 105 disposed so as to surround polarization coil 104 for focusing the electron beam.
As shown in FIG. 7, target portion 106 includes a glass substrate 1, a transparent conductive film 2 formed on the surface of glass substrate 1 and having a target power source 108 connected thereto, a hole injection stop layer 3 of cerium dioxide (CeO.sub.2) and germanium dioxide (GeO.sub.2) formed on the surface of transparent conductive film 2, a photoelectric converting layer 24 of amorphous selenium (Se) formed on the surface of hole injection stop layer 3, and an electron injection stop layer 7 of antimony trisulfide (Sb.sub.2 S.sub.3) formed on the surface of photoelectric converting layer 24. Hole injection stop layer 3 serves to prevent injection of holes from transparent conductive film 2 to photoelectric converting layer 24. Electron injection stop layer 7 serves to prevent injection of electrons of the electron beam to photoelectric converting layer 24.
Referring to FIGS. 6 and 7, operation of the conventional camera tube will be described.
Transparent conductive film 2 is fixed at a predetermined positive potential Vt by target power source 108. When light A is incident on one pixel in this state, electron/hole pairs are generated in a portion corresponding to the pixel in photoelectric converting layer 24. Out of electrons and holes generated in photoelectric converting layer 24, electrons move to the side of transparent conductive film 2, while holes move to the side of electron injection stop layer 7. As a result, the potential on the side of electron injection stop layer 7 increases.
On the other hand, on the surface of electron injection stop layer 7, an electron beam B is scanned. When the electron beam B comes onto the pixel having the potential at the side of electron injection stop layer 7 increased by incidence of light, a charging current flows in order to cancel the potential increase of the pixel. The charging current is taken out as a signal output.
In the above-described operation, by setting high the voltage of target power source 108, a high electric field is applied to photoelectric converting layer 24 of amorphous selenium. As a result, holes generated by incidence of light A causes an avalanche multiplication phenomenon (avalanche phenomenon), so that the number of holes increases. As a result, the number of holes reaching electron injection stop layer 7 increases, causing quantum efficiency as seen from the outside to exceed 1 into a value of 10 to 1000. It should be noted that quantum efficiency 1 is a state where one electron/hole pair is generated per photon. More specifically, quantum efficiency 10 corresponds to generation of 10 electron/hole pairs per photon. As described above, by applying a high electric field to photoelectric converting layer 24 of amorphous selenium, quantum efficiency has conventional been increased, so that sensitivity in the camera device has been made high.
However, photoelectric converting layer 24 of amorphous selenium shown in FIG. 7 had the following problems.
FIG. 8 is a graph of spectral sensitivity characteristics of the camera tube in which amorphous selenium shown in FIG. 7 is used as a photoelectric converting layer. The abscissa shows the wavelength, and the ordinate shows the quantum efficiency. Referring to FIG. 8, in the camera tube using the photoelectric converting layer of amorphous selenium, the sensitivity (quantum efficiency) decreases rapidly at the wavelength of 450 nm or more. Therefore, the sensitivity was deteriorated at the wavelength on the red side (approximately 600 to 700 nm).
In order to enhance the sensitivity in a visible light region (approximately 400 to 700 nm), a camera device using a photoelectric converting layer of amorphous silicon has been proposed. FIG. 9 is a graph of spectral sensitivity characteristics of the camera device using amorphous silicon as a photoelectric converting layer. The abscissa shows the wavelength, and the ordinate shows the relative sensitivity. FIG. 10 is a graph of spectral sensitivity characteristics in which the relation between the wavelength and the relative sensitivity shown in FIG. 9 is converted into the relation between the wavelength and the quantum efficiency. As shown in FIG. 9, by using the photoelectric converting layer of amorphous silicon, the relative sensitivity in the visible light region is improved. However, as shown in FIG. 10, in the camera device using the photoelectric converting layer of amorphous silicon, only lower quantum efficiency is obtained compared to the quantum efficiency of the photoelectric converting layer of amorphous selenium shown in FIG. 8. More specifically, it is difficult to prevent injection of electrons and holes in the photoelectric converting layer of amorphous silicon because of its material factor, whereby a high electric field cannot be applied to the photoelectric converting layer, causing the effect of an avalanche multiplication phenomenon to decrease. As a result, the camera device having favorable quantum efficiency (multiplication characteristics) could not be obtained.
As one example of the camera tube, a solid state image sensing device has conventionally been known. FIG. 11 is a cross sectional view of a conventional solid state image sensing device using amorphous selenium as a photoelectric converting layer. This is shown in, for example, Proceedings of National Conference of the Institute of Television Engineers of Japan, 1989, p. 41, and IEEE International Solid-State Circuit Conference, 1990, pp. 212 and 213. Referring to FIG. 11, the conventional solid state image sensing device using a photoelectric converting layer of amorphous selenium includes a p-type semiconductor substrate 8, an element isolation oxide film 10 formed in a predetermined region on the main surface of p-type semiconductor substrate 8 for isolating elements, an n-type signal charge storage region 9 enclosed by element isolation oxide film 10 and formed on the main surface of p-type semiconductor substrate 8, an interlayer oxide film 11 formed so as to cover p-type semiconductor substrate 8 and having a contact hole on n-type signal charge storage region 9, an interconnection layer 14 electrically connected to n-type signal charge storage region 9 in the contact hole of interlayer oxide film 11 and formed so as to extend along interlayer oxide film 11, an interlayer oxide film 12 formed on interconnection layer 14 and interlayer oxide film 11 and having a contact hole in a predetermined region on interconnection layer 14, an interconnection layer 15 electrically connected to interconnection layer 14 in the contact hole of interlayer oxide film 12 and formed so as to extend along interlayer oxide film 12, an interlayer oxide film 13 formed on interconnection layer 15 and interlayer oxide film 12 and having a contact hole in a predetermined region on interconnection layer 15, a pixel electrode 16 for one pixel electrically connected to interconnection layer 15 in the contact hole of interlayer oxide film 13 and formed so as to extend along the surface of interlayer oxide film 13, an electron injection stop layer 17 of arsenic trisulfide formed on pixel electrode 16 and interlayer oxide film 13, a photoelectric converting layer 24 of amorphous selenium formed on electron injection stop layer 23, a hole injection stop layer 27 of cerium dioxide formed on photoelectric converting layer 24, and a transparent conductive film 2 formed on hole injection stop layer 27.
In operation of the solid state image sensing device having the above-described structure, light is incident from above with transparent conductive film 2 therebetween. The light is directed to photoelectric converting layer 24 of amorphous selenium. By direction of the light, electron/hole pairs are generated in photoelectric converting layer 24. Out of the generated electrons and holes, the holes move to the side of electron injection stop layer 23, while the electrons move to the side of hole injection stop layer 27. The holes which move to the side of electron injection stop layer 23 are stored in n-type signal charge storage region 9 through pixel electrode 16, interconnection layer 15 and interconnection layer 14. The holes stored in n-type signal charge storage region 9 are read out as a signal.
The solid state image sensing device using amorphous selenium as a photoelectric converting layer also had problems similar to those of the camera tube using amorphous selenium as a photoelectric converting layer, shown in FIGS. 6 and 7. More specifically, in the solid state image sensing device using amorphous selenium as a photoelectric converting layer, the quantum efficiency (sensitivity) rapidly decreases at the wavelength of 450 nm or more. As a result, the sensitivity of the wavelength on the red side (approximately 700 nm) was deteriorated.
As described above, it was conventionally difficult to improve spectral sensitivity characteristics in the visible light region (especially the region on the red side) in the camera device using amorphous selenium as a photoelectric converting layer, because the quantum efficiency rapidly decreased in the visible light region (especially the region on the red side). Although the sensitivity was improved in the image sensing device using amorphous silicon as a photoelectric converting layer, it was difficult to obtain favorable quantum efficiency (multiplication characteristics) over the entire visible light region. As a result, it was difficult to improve spectral sensitivity characteristics in the visible light region.